The sermon addresses the root of sin by examining Jesus' teaching in Matthew 5 that lustful intent constitutes adultery in the heart. It defines lust as an evil desire that rebels against God's will, distinguishing between neutral desires and those hijacked by sin. The speaker emphasizes the reality of hell and the serious consequences of unrepentant sin to urge immediate spiritual vigilance. A practical battle plan is offered, focusing on setting one's mind on heavenly things and putting on godly virtues like love and humility. Ultimately, believers are called to actively pursue what is good and pure, allowing God's word to dwell richly within them to overcome earthly cravings.